Magnetic materials fall generally into two classes, magnetically hard substances which may be permanently magnetized, and magnetically soft substances of high permeability. It is with the latter that the present invention is concerned. Permeability is a measure of the ease with which a magnetic substance can be magnetized and it is given by the ratio B/H, H representing the magnetic force necessary to produce the magnetic induction B. In most power applications, such as transformers or inductors, motors, generators and relays, iron is used as the magnetic material and high permeability together with low losses are highly desirable.
When magnetic material is exposed to a rapidly varying field, it is subject to hysteresis losses and eddy current losses. The hysteresis loss results from the expenditure of energy to overcome the magnetic retentive forces within the iron. The eddy current loss results from the flow of electric currents within the iron induced by the changing flux. Hysteresis and eddy current losses together make up the core of iron losses in a transformer or electromagnetic device. The conventional practice in making magnetic cores for use in transformers has been to form a laminated structure by stacking thin ferrous sheets. The sheets are oriented parallel to the magnetic field to assure low reluctance. They may be varnished or otherwise coated to provide insulation between sheets which prevents current from circulating between sheets and this keeps eddy current losses low. Conventional laminated transformers and inductors require many different operations in their manufacture.
The use of sintered powder metal avoids the manufacturing burden inherent in laminated structures but, due to the high core losses, has generally been restricted to applications involving direct current operation such as relays. Alternating current applications require that the iron particles be insulated from one another in order to reduce eddy current losses. Powder cores made of magnetic iron oxide and other metal oxides combined to form a ceramic (ferrite), or of iron powder dispersed in plastic material, are used in high frequency and signal level circuits. To our knowledge metal powder cores have not heretofore been used for power transformers or motors due to their low flux carrying capability.
In a typical reactor ballast for a high intensity discharge (HID), or for any arc discharge lamps using a laminated core, an air gap whose length is from about 1% to 3%, more commonly 1% to 2%, of the magnetic circuit is provided. If iron powder is to be used for the magnetic core in such an application, the particles must be insulated from one another with no more than 1% to 3% spacing between particles. When raw iron powder is compressed even up to 100 tons per square inch and not sintered, the density remains 1% or 2% below the true density of solid iron, probably because of residual tiny crevices or interstices which remain empty. This means that the iron powder must be compressed to about 90% of theoretical density or better in order to have a distributed insulation-containing air gap not exceeding 3% in each of the three orthogonal directions one of which is that of the flux path.
Various attempts have been made in the past to form high density magnetic cores with the desired properties by compacting steel powder coated with insulating material. U.S. Pat. No. 3,245,841 describes a process for producing high resistivity steel powder by treating the powder with phosphoric acid and chromic acid to provide a surface coating on the steel particles consisting principally of iron phosphate and chromium compounds. U.S. Pat. No. 3,725,521--Ebling, describes another process for the same purpose and in which the steel particles are coated with a thermosetting resin such as a silicone resin. The same patent proposes loading the resin with an inorganic filler of smaller particle size than the steel powder, such as quartz, kaolin, talc, calcium carbonate and the like. U.S. Pat. No. 4,177,089--Bankson, proposes a blend of iron and iron-silicon aluminum alloy particles which are coated with alkali metal silicate, clay and alkaline earth metal oxide. None of these prior proposals has succeeded in producing a magnetic core of the required density and having a resistivity high enough that the core losses are not substantially greater than those occurring in the conventional laminated cores. Up to the present time there has been no commercial use of pressed iron powder cores for HID lamp ballasts.